Abstract
Sustainability has become one of the most important goals when optimizing traffic signals. This goal is achieved through utilizing various objective functions to reduce sustainability metrics (e.g., fuel consumption and emissions). However, most available objective functions do not distinguish between the reduction mechanism of various types of emissions. Further, such functions do not consider the compound impact of multiple operational conditions (e.g., road gradient) influencing emissions on the optimized signal plans. This study derives a new Environmental Performance Index representing a surrogate measure for emission estimates that can be used as an objective function in signal timings optimization to reduce emissions under various operational conditions. The Environmental Performance Index is a linear combination of delays and stops. The key factor of the Environmental Performance Index is the emissions-based stop penalty, which represents an emission stop equivalency measured in seconds of delay. This study also uses traffic simulation and emission models to investigate the compound impact of several operational conditions on the stop penalty. Results show that the stop penalty varies significantly with all the investigated conditions and that the stop penalty is unique for different types of emissions. These findings may have significant implications on the current practice of sustainable signal timing optimization.
Highlights
Various emission types are determinantal to public health and the environment of the Earth as a whole
Vehicles do not always need to be moving to release emissions [32,33]; vehicular emissions can be classified according to the vehicle operating mode in which emissions are emitted into three categories: 1—Evaporative “non-tailpipe” emissions are mainly driven by diurnal fuel evaporation, residual engine heat following vehicle operation inducing hot soak emissions, and running evaporative loss emissions that occur while vehicles are running [32]; 2—Refueling emission are volatile organic compound (VOC) vapor and entrained droplets displaced from the fuel tank ullage [33]; 3—Tailpipe “exhaust”
It is expected that the same investigation results in other regions would not deviate significantly from the results presented in this paper, we recommend using a relevant emissions model to the area of interest when computing the emissionsbased stop penalty
Summary
Various emission types are determinantal to public health and the environment of the Earth as a whole. The general practice of optimizing signals to minimize delay does not necessarily minimize extra stops; emissions could increase [6,7,8]. In some studies, the balancing between delay and stops has shifted gradually to a tradeoff process between delay and sustainable metrics (e.g., fuel consumption and emissions) [6,7,9,10,11,12,13]. This study contributes significantly to the research on sustainable signal timing optimization by introducing a family of implementable objective functions to minimize emissions. The second part reviews the most notable studies concerning vehicular emissions and the optimization of traffic signals to minimize emissions and fuel consumption. The conclusions summarize the study, mention its limitations, and provide insights for future research
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